The biblical category for demonic spirits, especially as encountered in the Gospels and Acts. The Greek akatharton pneuma (unclean spirit) appears throughout the Synoptic Gospels: the demoniac of Mark 1:23-28, the Gerasene demoniac of Mark 5, the Syrophoenician woman's daughter (Mark 7:25-30), and many others. Unclean here is not merely dirty; it is the ritual-and-moral opposite of holy. Demons are unclean because they are spirits of rebellion against the Holy God. Christ's ministry of casting out unclean spirits demonstrated His authority over the entire spiritual realm (Mark 1:27: even the unclean spirits, and they do obey him) and signaled the in-breaking of the kingdom (Matt 12:28). Matthew 12:43-45 gives the warning: an unclean spirit cast out without the house being filled by the Spirit of God may return with seven worse than itself. Deliverance is real; the only durable replacement is the Holy Spirit's indwelling.